Albert Einstein first published his theory of special relativity in 1905; his work on general relativity followed in 1915. By 1923, Premier Productions made a silent film to explain the most salient bits. Though it’s entirely silent (and, of course, 100 years old), it’s a thoroughly lucid way to understand Einstein’s most important work—with some great vintage animation as well.
The theory of special relativity revolutionized the ways that scientists study and understand the universe. It introduced the famous equation E = mc2, which stands for “energy = mass x the speed of light squared.” Fundamentally, special relativity proposes that mass and energy are the same thing but in different forms. Each can be converted to the other form under specific conditions.